SC-FDMA (single-carrier frequency-division multiple access) having a small PAPR (peak-to-average power ratio) and providing high power utilization efficiency for terminals has been adopted for 3GPP LTE (3rd generation partnership project long-term evolution, hereinafter simply referred to as “LTE”) uplink. In the LTE uplink, SRSs (sounding reference signals) are used to obtain CSI (channel state information) including various kinds of information such as path loss and channel frequency response or the like (e.g., see Non-Patent Literature 1).
Each terminal transmits SRSs at periods set beforehand using time and frequency resources allocated beforehand. A base station measures uplink CSI based on SRSs periodically received from each terminal in a cell and references the CSI of each terminal to thereby perform frequency scheduling (resource allocation in frequency domain) of a PUSCH (physical uplink shared channel).
A wide-band LTE uplink becomes a frequency selective fading channel whose gain significantly differs depending on its frequency. Therefore, the base station allocates a PUSCH to a frequency resource having a large gain, and can thereby maintain high channel quality.
In order for the base station to perform frequency scheduling of a PUSCH, the terminal needs to transmit SRSs in all available bands.
When the terminal is located in the vicinity of the base station, the terminal transmits SRSs having a wide band and small power density. The base station can measure wide-band CSI necessary for scheduling of the PUSCH by receiving only one SRS.
On the other hand, when the terminal is located far from the base station, for example, at a cell edge, a path loss in a propagation path is large, and power of a signal transmitted from the terminal thereby significantly attenuates by the time the signal arrives at the base station. Therefore, in order for the base station to obtain desired receiving quality, the terminal is required to increase transmission power.
However, transmission power of the terminal has an upper limit and increasing a power density in a wide band may cause the transmission power to exceed the upper limit value. For this reason, the terminal sends SRSs having a large power density in a narrow band (a bandwidth resulting from dividing the whole bandwidth by n, n being an integer equal to or greater than 2) a plurality of times while changing a band (frequency hopping). This allows the base station to receive a plurality of SRSs, temporally accumulate the SRSs and thereby measure CSI of the whole bandwidth necessary for PUSCH scheduling.
Note that LTE-Advanced Release 10 (hereinafter, described as “Rel. 10”) which is an evolved version of LTE has introduced A-SRS (aperiodic-SRS) in addition to periodically transmitted SRS (periodic-SRS, hereinafter, referred to as “P-SRS”) (e.g., see Non-Patent Literature 2). An A-SRS is transmitted from the terminal only once in response to a transmission request transmitted from the base station. Since the base station needs to transmit a transmission request to the terminal only when the base station desires to obtain CSI of a predetermined band, operation with minimized resource consumption is possible in Rel. 10.
In Release 11 which is the next LTE-Advanced version (hereinafter, described as “Rel. 11”), in a heterogeneous cell network (HetNet) as shown in FIG. 1, where there are a plurality of base stations having different cover areas, studies are being carried out on CoMP (coordinated multi-point) transmission/reception in which a plurality of base stations carry out coordinated transmission/reception (e.g., see Non-Patent Literature 3). HetNet is made up of a macro base station and pico base stations. The macro base station is a base station (node) having large transmission power and coverage, and the pico base station is a base station (node) having small transmission power and coverage. In Rel. 11 in particular, since a base station located in the vicinity of a terminal can perform reception on the uplink, it is possible to achieve high quality transmission while reducing required transmission power in the terminal compared to Rel. 10 or earlier in which only a macro base station exists.
Here, in HetNet, since transmission/reception is performed at multiple points located at greatly varying distances, it is necessary to appropriately select a base station that performs transmission/reception with a terminal (hereinafter, a base station that carries out transmission/reception to/from a terminal is called “transmission/reception participating base station”) and also switch between transmission/reception participating base stations as appropriate as the terminal moves. Selection of the transmission/reception participating base station is done by the macro base station.
Studies are being carried out on the use of reference signals (CRS, CSI-RS, and SRS) transmitted on the uplink and downlink for selection and switching of transmission/reception participating base stations. When a CRS or CSI-RS transmitted on the downlink is used, the terminal measures CSI up to each base station and feeds back the measurement result using the uplink. The macro base station determines a transmission/reception participating base station based on the fed back CSI. On the other hand, when an SRS transmitted on the uplink is used, the base station can directly measure CSI using the SRS transmitted from the terminal. Therefore, it is possible to reduce the amount of information fed back from the terminal to the base station compared to a case where a CRS or CSI-RS is used.
As described above, in Rel. 11 or later in which CoMP is introduced, there is a high possibility that selection of a transmission/reception participating base station using SRSs may be adopted. In this case, an SRS is used for two applications: (1) PUSCH frequency scheduling, and in addition (2) selection of a transmission/reception participating base station.